In many types of minority carrier semiconductor devices, for example, photodetectors, solar cells, light emitting diodes and laser diodes, device performance may be limited by surface recombination of carriers. Surface recombination of carriers is generally undesirable and may limit device efficiency and performance for at least two reasons. First, the efficiency of minority carrier devices is reduced because carriers are lost in the recombination process and are, therefore, not collected and, second, surface recombination inevitably leads to some heating of the semiconductor device, and as the device temperature rises, the device is more likely to fail.
The probability of surface recombination of carriers is proportional to what is termed the surface recombination velocity. Reduction of the surface recombination velocity can yield substantial improvements in efficiency for many devices. This is true for direct bandgap solar cells where most carriers are generated close to the surface. For example, theoretical calculations of the internal spectral response for GaAs P/N solar cells having a junction depth of 0.5 .mu.m show that a reduction in the surface recombination velocity from 10.sup.6 cm/sec to 10.sup.4 cm/sec increases the device efficiency by a factor of approximately 2. Further reductions in surface recombination velocity, however, yield only a small improvement in efficiency. The effect of surface recombination velocity on efficiency is discussed by H. J. Hovel in Semiconductors and Semimetals, Vol. 11, Solar Cells, Academic Press, New York, 1975, at pp. 28-29.
Several techniques have been used in attempts to reduce the surface recombination velocity which is proportional to the number of surface trapping centers per unit area at the boundary region. For example, a lattice matched heterojunction formed by growing a layer of Ga.sub.1-x Al.sub.x As on the air-exposed surface of GaAs reduces the recombination velocity. The recombination velocity after such a layer has been grown is typically 450 cm/sec. This is approximately 3 orders of magnitude smaller than the surface recombination velocity at the GaAs-air interface. The surface state density may be altered by well known surface treatments such as the growth of native oxides and deposition of SiO.sub.2 or silicon nitride. However, these surface treatments have not been shown to decrease the surface recombination rate, although they do alter the surface state density. Thus, growth of a lattice matched heteroepitaxial layer appears to be the only technique presently available which reduces the surface recombination velocity. However, for many applications, growth of a heteroepitaxial layer, resulting in the formation of a lattice matched heterojunction, is either not practical or desirable, and other techniques which reduce the surface recombination velocity would be useful.
Surface treatment of one GaAs device has been shown to greatly improve device efficiency. Chemical treatment of the GaAs electrode in a semiconductor-liquid junction solar cell has been shown to increase device efficiency. In Applied Physics Letters, 33, p. 521 (1978), an efficiency of approximately 12 percent for a solar cell having a chemically treated electrode was reported. An efficiency of less than 10 percent was obtained for a similar cell without such electrode surface treatment. The particular device described had a fraction of a monolayer of ruthenium on the GaAs electrode surface that contacted an aqueous selenide/polyselenide solution. It was hypothesized in the article that the ruthenium on the GaAs electrode surface altered the surface states initially within the GaAs bandgap in such a way that recombination was reduced and efficiency increased.
The article described only devices having chemically treated GaAs electrodes contacting aqueous selenide/polyselenide solutions. There appears to be no way to accurately predict whether such chemical treatment of a gallium arsenide surface would reduce recombination and enhance device efficiency if the chemically treated semiconductor surface contacted any other material such as air, another gas or a metal.